It is known in the art relating to the manufacture of electrical semiconductor devices that a semiconductor film may be deposited on the surface of a substrate to produce useful semiconducting devices such as diodes, solar cells and transistors. For example, in the article, W. E. Spear et al, "Amorphous Silicon PN Junction", Applied Physics Letters, Vol. 28, pp. 105-107 (1976), it is shown that doped amorphous silicon layers that are produced from glow discharge plasmas of monosilane (SiH.sub.4) may be deposited so as to form PN junctions with useful electrical rectification characteristics. Of course, other potential applications for deposited silicon films are known to those skilled in the art.
In the U.S. Pat. to Carlson, No. 4,064,521, it is indicated that an amorphous silicon semiconducting film may be formed on a substrate by a capacitive glow discharge in a monosilane (SiH.sub.4) gas. It is further indicated that the amorphous silicon film that is produced possesses characteristics that are ideally suited for a photovoltaic device and that are superior to the characteristics of a crystalline silicon film. For example, the optical absorption of the amorphous silicon is superior to the absorption of single crystalline silicon over the visible light range. Also, the electron lifetime in amorphous silicon that is produced by the glow discharge method is substantially longer than the electron lifetime of amorphous silicon that is formed by either sputtering or evaporation.
In the article by M. H. Brodsky, "Plasma Preparations of Amorphous Silicon Films", Thin Solid Films, Vol. 50, pp. 57-67 (1978), it is indicated that if a relatively high gas pressure is used in the glow discharge of monosilane, the deposited silicon film will polymerize, thereby degrading the amorphous silicon that is produced. Accordingly, in the process of Carlson, the pressure must be maintained at a relatively low level during the glow discharge process in order to ensure that amorphous silicon having the desired photoelectric properties will be produced.
However, the deposition parameters of a chemical vapor deposition process, such as the glow discharge process of Carlson, determine the rate at which a material is deposited on a substrate. More particularly, it has been determined that, in general, the rate of deposition will be increased if the pressure is increased during the deposition process. Thus, if it is necessary to reduce the pressure to produce a particular deposition product, as is suggested in Carlson, the reduced pressure will result in a corresponding reduced rate of deposition and an associated decrease in the efficiency of the deposition process.
The slow rate of deposition also, apparently, causes a strain to appear at the interface between the deposited film and the supporting substrate. The strain tends to warp or distort the substrate and also limits the thickness of the film that may be deposited on the substrate. Moreover, a reduced rate of deposition often results in the deposition of a film that has a rather rough surface, since the slow rate of deposition makes the deposition process more sensitive to physical factors that operate to vary the rate of growth at different locations on the substrate. The roughness of the film increases the probability that the film will break down in response to applied operational voltages and, also, the rough surface may cause nonuniform electrical fields to be produced if the film is employed in a semiconductor device.
It is shown in the U.S. Pat. to Androshuk et al, No. 3,424,661, that thin films of the oxides, carbides, borides and nitrides of silicon may be deposited on a substrate by a DC reactive plasma technique wherein monosilane (SiH.sub.4) is combined in a plasma with an associated reacting gas to deposit a silicon compound on a heated substrate. Disilane (Si.sub.2 H.sub.6) and trisilane (Si.sub.3 H.sub.8) are purported to be chemically equivalent to monosilane.
In the glow discharge process of Androshuk et al, it is necessary to establish particular levels of temperature, pressure and DC excitation power to ensure that a film having favorable protective or other insulating properties is deposited. Accordingly, as indicated at Col. 4, lines 10-11 of Androshuk et al, a gas pressure of from 0.1 torr to 10 torr must be applied in the glow discharge apparatus to form a plasma that is sufficiently dense to be useful in the deposition process.
It is indicated at Col. 4, lines 42-49 of Androshuk et al that amorphous silicon nitride films have desirable insulating or protective properties and may be produced if a particular, relatively low temperature range is maintained during the glow discharge process. It is further indicated that as the deposition temperature rises outside the accepted range, the film becomes increasingly crystalline, thereby losing the favorable properties associated with an amorphous structure. Thus, insulating or protective films may be deposited in accordance with the Androshuk process at a relatively high pressure of from 0.1 torr to 10 torr and, if an amorphous silicon nitride film is desired, a relatively low deposition temperature must be maintained.
In the paper by R. A. Street, J. C. Knights and D. K. Biegelsen, "Luminescence Studies of Plasma-Deposited Hydrogenated Silicon", Physical Review, Vol. 18, pp. 1885-1891 (1978), it is stated that favorable semiconducting properties of deposited silicon films are associated with the luminescence of the films, and the luminescence of the films decreases with increasing pressure and RF excitation power. Thus, in prior art deposition systems, the deposition pressure and power have been maintained at relatively low levels in order to provide superior semiconducting properties for a deposited film. However, as indicated above, lower deposition pressures tend to slow down the rate of deposition and thereby decrease the efficiency of the deposition process.
Accordingly, it is a primary object of the invention to provide an improved process for rapidly and efficiently depositing a film having superior photoelectronic, semiconducting or other electrical properties.
Another object of the invention is to provide such a deposition process wherein the film is deposited on a substrate by a glow discharge in a silane of higher order than monosilane, the higher order silanes defined, in part, by the expression Si.sub.n H.sub.y, where n is equal to or greater than two.
A further object of the invention is to provide a glow discharge deposition process wherein the rate of the deposition is sufficient to ensure that there is a reduced strain or no strain at the interface between a deposited film and an associated supporting substrate.
Another object of the invention is to provide a deposition process wherein the deposited film has a relatively smooth surface.
A further object of the invention is to provide a deposition process wherein an amorphous silicon layer is rapidly formed on a substrate in response to a relatively low temperature and pressure and a relatively low excitation power in a glow discharge apparatus.
Another object of the invention is to provide a deposition process wherein a layer of hydrogenated amorphous silicon is deposited by a glow discharge in a silane gas of higher order than monosilane.